Abstract: In the present study 0, 0.5, 1.0, 2.0, 4.0 and 8 mL of Effective Microorganisms (EM) concentrations were diluted with 1000 mL of tap-water and at each concentration was used to mix with Pleurotus ostreatus solid culture. Growth of P. ostreatus mycelia were scrutinized during 5 weeks of incubation period. Rapid growth of P. ostreatus mycelia were revealed in the EM mixed P. ostreatus solid culture. Twenty percent contamination were found in the EM unmixed P. ostreatus growth substrates while 0-5% were detected in the EM mixed P. ostreatus solid culture. Averages of total mushroom weight and number of oyster mushrooms (OMs.) were high when watering P. ostreatus with the tap-water instead of EM solution. 0.43-0.63 kg of the total mushroom weight and 69-110 of OMs. were detected per block. Mean total mushroom weight and number of OMs. per block was 2.0 to 4.0 times higher in the EM mixed than unmixed P. ostreatus growth substrates. However, it was found that irrigation of OM cultures using ordinary tap water and irrigation using EM dissolved in tap water, gave almost insignificant differences in average weight, width and height of OMs. at a confidence level of 95%, where the average weight, width and height of each OM was found to be approximately 6.26 g, 5.96 and 7.89 cm, respectively.